Tyre tread having a composition containing a xanthan gum powder

ABSTRACT

The invention relates to a tyre tread having a composition based on at least one diene elastomer, one reinforcing filler and one vulcanization system, characterized in that the composition comprises a xanthan gum powder in a proportion of 10 to 40 pce. 
     The invention also relates to a process for obtaining a rubber composition for a tread based on at least one diene elastomer, one reinforcing filler and one vulcanization system which comprises a first phase of thermomechanical kneading of the constituents of the composition, with the exception of the vulcanization system, characterized in that the composition comprises a xanthan gum powder in a proportion of 10 to 40 pce and in that the xanthan gum powder is incorporated during the first kneading phase.

The invention relates to rubber compositions for tyre treads and moreparticularly to rubber compositions which can be used in the treads of“winter” tyres, that is to say of tyres which are particularly effectiveon wet ground or ground covered with snow or melting ice, that is to sayfor temperatures of between −10° C. and 0° C., to preferably between −5°C. and 0° C.

Different solutions exist for making it possible to obtain good tyregrip, even on ground covered with snow or ice. Thus, studded tyres meetthese criteria and make it possible to effectively resist sliding onice; however, their very strong abrasive action on the ground surfacingitself is gradually resulting in them being banned.

Manufacturers have thus looked for other types of solutions consistingin modifying the rubber composition itself of tyre treads.

Some solutions consist, for example, in incorporating water-solublepowders in the constituent rubber mixture of the tread. Such powdersdissolve on contact with snow or molten ice, which makes possible, onthe one hand, the creation at the surface of the tyre tread ofporosities capable of improving “the attachment” of the tread to theground and, on the other hand, the creation of grooves acting aschannels for discharge of the liquid film created between the tyre andthe ground.

Thus, the publication JP-3-159803 describes tyre treads, the rubbercomposition of which comprises from 3 to 25 pce (pce meaning parts byweight per 100 parts of elastomers) of polysaccharide powder, such assodium carboxymethylcellulose powder, or of vinyl alcohol powder with aparticle size of between 20 and 600 μm and with a solubility at 0° C. ofat least 5 g per 100 ml of water, which solubility is obtained on mixingfor a few tens of seconds.

In the same way, the publication EP 0 940 435 describes a rubbercomposition for a tread comprising from 1 to 25 pce of modifiedpolyvinyl alcohol with a particle size of between 10 and 100 μm and asolubility at 0° C. of 1 g per 200 ml of water for a very short mixingtime, that is to say of the order of one minute.

Finally, the publication JP200211203 describes a rubber composition fora tread comprising from 3 to 40 pce of starch powder, the particles ofwhich have a size of greater than or equal to 20 μm and a solubility of1 g in 100 ml of water at 10° C., achieved after mixing for 3 minutes.

In all these examples, it is clear that the solubility at very lowtemperature and in a very short time of the powder used in the rubbercomposition is an essential factor in the satisfactory operation of thetread produced. This is because it is specified, in the abovementioneddocuments, that, if the powder is not soluble under the conditions ofuse of the tyre, an adhesive layer is no longer formed at the surface ofthe tread and that consequently the resistance to sliding isdetrimentally affected. Unfortunately, this good performance with regardto grip is accompanied by very poor results in terms of wear resistance,which can be monitored by the deterioration in the rubber properties,such as stiffness, elongation and tensile strength properties, andhysteric losses, which render these solutions unusable industrially.

The Applicant Company has discovered, surprisingly, that theintroduction into the constituent rubber composition of the tread of aspecific polysaccharide powder which is insoluble in water in a veryshort time (of the order of one minute) and at very low temperature (ofthe order of 0°) nevertheless makes it possible to improve the behaviorof the tyres in terms of resistance to sliding and of grip on snowy orfrozen ground without the slight fall in wear resistance accompanyingthis modification having a prohibitory effect on the industrial use ofsuch tyres.

In that which follows, tyre tread will denote the complete tread or aportion of the latter, in particular when it is composed of severallayers, in contact with the ground.

The invention thus relates to a tyre tread having a rubber compositionbased on at least one diene elastomer, one reinforcing filler and onevulcanization system, characterized in that the composition comprises axanthan gum powder in a proportion of 10 to 40 pce and preferably of 15to 30 pce.

More particularly, the size of the particles of the xanthan gum powderis between 10 and 500 μm, preferably between 50 and 300 μm.

According to one characteristic of the invention, the xanthan gum powderis used as a blend with one or more other polysaccharide powders and inparticular with a guar gum powder.

Advantageously, the diene elastomer is predominantly, for more than 50pce, an isoprene elastomer.

According to one embodiment of the invention, the reinforcing fillerpredominantly comprises carbon black.

According to another embodiment, the reinforcing filler predominantlycomprises silica or comprises a blend of carbon black and silica.

Another subject-matter of the invention is a process for obtaining arubber composition for a tread based on at least one diene elastomer,one reinforcing filler and one vulcanization system which comprises afirst phase of thermomechanical kneading of the constituents of thecomposition, with the exception of the vulcanization system,characterized in that the composition comprises a xanthan gum powder ina proportion of 10 to 40 pce, preferably of 10 to 30 pce, and in thatthe xanthan gum powder is incorporated during the first kneading phase.

Preferably, the first phase of the process is carried out in two stages:a first stage of kneading the combined constituents of the composition,with the exception of the xanthan gum powder, and a second stage inwhich the xanthan gum powder is incorporated.

According to one embodiment of the invention, the first stage of thefirst kneading phase is carried out at a temperature of between 110° C.and 190° C. and preferably between 130° C. and 180° C.; the second stageof the first kneading phase is carried out at a temperature of less thanor equal to 130° C., after cooling on conclusion of the first stage,preferably to a temperature of less than or equal to 100° C.

Advantageously, the second phase of the process is carried out at atemperature of between 60° C. and 100° C.

A further subject-matter of the invention is a tyre comprising a tread,the rubber composition of which is based on at least one dieneelastomer, one reinforcing filler and one vulcanization system,characterized in that the composition comprises a xanthan gum powder ina proportion of 10 to 40 pce, and particularly a winter tyre comprisingsuch a tread.

A further subject-matter of the invention is a tyre intended to equip a“heavy-duty” vehicle (that is to say, underground, bus, heavy roadtransport vehicles (lorries, tractors, trailers) or off-road vehicles),planes, earthmoving equipment, heavy agricultural vehicles or handlingvehicles, comprising a tread in accordance with the invention.

Finally, a subject-matter of the invention is a tyre intended to equip apassenger vehicle comprising a tread in accordance with the invention.

A person skilled in the art usually denotes by “polysaccharide gums” anypolysaccharide or its derivatives which, dispersed in water, producehighly viscous gels or produce solutions.

Xanthan gum is included among polysaccharide gums; it involves dietaryfibres “regarded” as thickeners and gelling agents which are insolublein water; specifically, they form a gel on contact with water.

The rubber compositions according to the invention are based on thefollowing constituents: at least one diene elastomer, one reinforcingfiller and one crosslinking system.

The expression “composition based on” should be understood as meaning acomposition comprising the mixture and/or the in situ reaction productof the various base constituents used, some of these constituents beingable to react and/or being intended to react with one another, at leastpartially, during the various phases of manufacture of the compositionor during the subsequent curing.

“Diene” elastomer or rubber is understood to mean, in a known way, anelastomer resulting at least in part (i.e., a homopolymer or copolymer)from diene monomers (monomers carrying two conjugated or nonconjugatedcarbon-carbon double bonds).

Generally, “essentially unsaturated” diene elastomer is understood hereto mean a diene elastomer resulting at least in part from conjugateddiene monomers having a level of units of diene origin (conjugateddienes) which is greater than 15% (mol %).

Thus it is, for example, that diene elastomers, such as butyl rubbers orcopolymers of dienes and of α-olefins of EPDM (ethylene/propylene/dieneterpolymer) type do not come within the preceding definition and can inparticular be described as “essentially saturated” diene elastomers (lowor very low level of units of diene origin, always less than 15%).

In the category of “essentially unsaturated” diene elastomers, “highlyunsaturated” diene elastomer is understood to mean in particular a dieneelastomer having a level of units of diene origin (conjugated dienes)which is greater than 50%.

Given these definitions, essentially unsaturated diene elastomer capableof being used in the compositions in accordance with the invention isunderstood in particular to mean:

-   -   any homopolymer obtained by polymerization of a conjugated diene        monomer having from 4 to 12 carbon atoms;    -   any copolymer obtained by copolymerization of one or more        conjugated dienes with one another or with one or more        vinylaromatic compounds having from 8 to 20 carbon atoms.

The following are suitable in particular as conjugated dienes:1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C₁ to C₅alkyl)-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene,1,3-pentadiene or 2,4-hexadiene.

The following, for example, are suitable as vinylaromatic compounds:styrene, ortho-, meta-, or para-methylstyrene, the “vinyltoluene”commercial mixture, para-(tert-butyl)styrene, methoxystyrenes,chlorostyrenes, vinylmesitylene, divinylbenzene or vinylnaphthalene.

The copolymers can comprise between 99% and 20% by weight of diene unitsand between 1% and 80% by weight of vinylaromatic units. The elastomerscan have any microstructure which depends on the polymerizationconditions used, in particular on the presence or absence of a modifyingand/or randomizing agent and on the amounts of modifying and/orrandomizing agents employed. The elastomers can, for example, be block,random, sequential or microsequential blocks and can be prepared indispersion or in solution; they can be coupled and/or star-branched andalso functionalized with a coupling and/or star-branching orfunctionalization agent.

The following are preferably suitable: polybutadienes, in particularthose having a content of 1,2-units of between 4% and 80% or thosehaving a content of cis-1,4-units of greater than 80%, polyisoprenes,butadiene/styrene copolymers and in particular those having a styrenecontent of between 5% and 50% by weight and more particularly between20% and 40%, a content of 1,2-bonds of the butadiene part of between 4%and 65% and a content of trans-1,4-bonds of between 20% and 80%,butadiene/isoprene copolymers, in particular those having an isoprenecontent of between 5% and 90% by weight and a glass transitiontemperature (Tg) of −40° C. to −80° C., or isoprene/styrene copolymers,in particular those having a styrene content of between 5% and 50% byweight and a Tg of between −25° C. and −50° C.

In the case of butadiene/styrene/isoprene copolymers, those having astyrene content of between 5% and 50% by weight and more particularly ofbetween 10% and 40%, an isoprene content of between 15% and 60% byweight and more particularly between 20% and 50%, a butadiene content ofbetween 5% and 50% by weight and more particularly of between 20% and40%, a content of 1,2-units of the butadiene part of between 4% and 85%,a content of trans-1,4-units of the butadiene part of between 6% and80%, a content of 1,2-plus 3,4-units of the isoprene part of between 5%and 70% and a content of trans-1,4-units of the isoprene part of between10% and 50%, and more generally any butadiene/styrene/isoprene copolymerhaving a Tg of between −20° C. and −70° C., are suitable in particular.

Particularly preferably, the diene elastomer of the composition inaccordance with the invention is chosen from the group of the highlyunsaturated diene elastomers consisting of polybutadienes (BR),polyisoprenes (IR) or natural rubber (NR), butadiene/styrene copolymers(SBR), butadiene/isoprene copolymers (BIR), isoprene/styrene copolymers(SIR), butadiene/acrylonitrile copolymers (NBR), isoprene/styrenecopolymers (SIR), butadiene/styrene/isoprene copolymers (SBIR),butadiene/styrene/acrylonitrile copolymers (NSBR) or a mixture of two ormore of these compounds.

According to another preferred embodiment, the diene elastomer ispredominantly (for more than 50 pce) an isoprene elastomer. This is thecase in particular when the compositions of the invention are intendedto constitute, in the tyres, rubber matrices of certain treads (forexample for industrial vehicles).

“Isoprene elastomer” is understood to mean, in a known way, an isoprenehomopolymer or copolymer, in other words a diene elastomer chosen fromthe group consisting of natural rubber (NR), synthetic polyisoprenes(IR), the various copolymers of isoprene and the mixtures of theseelastomers. Mention will in particular be made, among isoprenecopolymers, of isobutene/isoprene copolymers (butyl rubber—IIR),isoprene/styrene copolymers (SIR), isoprene/butadiene copolymers (BIR)or isoprene/butadiene/styrene copolymers (SBIR). This isoprene elastomeris preferably natural rubber or synthetic cis-1,4-polyisoprene; use ispreferably made, among these synthetic polyisoprenes, of polyisopreneshaving a level (mol %) of cis-1,4-bonds of greater than 90%, morepreferably still of greater than 98%.

Nevertheless, such diene polymers can be used alone or as a blend withother elastomers conventionally used in tyres, such as essentiallysaturated diene elastomers, such as,

-   -   a ternary copolymer obtained by copolymerization of ethylene and        of an α-olefin having from 3 to 6 carbon atoms with a        nonconjugated diene monomer having from 6 to 12 carbon atoms,        such as, for example, elastomers obtained from ethylene and        propylene with a nonconjugated diene monomer of the        abovementioned type, such as in particular 1,4-hexadiene,        ethylidenenorbornene or dicyclopentadiene;    -   a copolymer of isobutene and of isoprene (butyl rubber or IIR)        and the halogenated, in particular chlorinated or brominated        (BIIR), versions of this type of copolymer;    -   or a copolymer of isobutene and of para-methylstyrene and the        halogenated, in particular chlorinated or brominated (BIMS),        versions of this type of copolymer.

Use may in particular be made, as reinforcing filler, of carbon black orsilica or other reinforcing fillers, alone or as a blend with oneanother.

More particularly, all carbon blacks, in particular blacks of the HAF,ISAF or SAF type, conventionally used in tyres and particularly in tyretreads are suitable as carbon blacks. Mention may be made, asnonlimiting examples of such blacks, of the N115, N134, N234, N330,N339, N347 or N375 blacks.

In the case of a silica, use is preferably made of a highly dispersibleprecipitated silica, in particular when the invention is made use of inthe manufacture of tyres exhibiting a low rolling resistance; mentionmay be made, as nonlimiting examples of such preferred highlydispersible silicas, of the Ultrasil 7000 and Ultrasil 7005 silicas fromDegussa, the Zeosil 1165 MP, 1135 MP and 1115 MP silicas from Rhodia,the Hi-Sil EZ150G silica from PPG, the Zeopol 8715, 8745 and 8755silicas from Huber or treated precipitated silicas, such as, forexample, the silicas “doped” with aluminum described in theabovementioned Application EP-A-0 735 088.

The vulcanization system proper is preferably based on sulphur and on aprimary vulcanization accelerator, in particular on an accelerator ofthe sulphonamide type. Additional to this base vulcanization system arevarious known vulcanization activators or secondary accelerators, suchas zinc oxide, stearic acid, guanidine derivatives (in particulardiphenylguanidine), and the like. The sulphur is used at a preferredlevel of between 0.5 and 10 pce, more preferably of between 0.5 and 5.0pce, for example between 0.5 and 3.0 pce, when the invention is appliedto a tyre tread. The primary vulcanization accelerator is used at apreferred level of between 0.5 and 10 pce, more preferably of between0.5 and 5.0 pce, in particular when the invention is applied to a tyretread.

It is obvious that the invention relates to the rubber compositionsdescribed above both in the “raw” state (i.e., before curing) and in the“cured” or vulcanized state (i.e., after crosslinking or vulcanization).

Of course, the compositions in accordance with the invention can be usedalone or as a blend (i.e., as a mixture) with any other rubbercomposition which can be used for the manufacture of tyres. The rubbercompositions in accordance with the invention can also comprise all or aportion of the usual additives generally used in elastomer compositionsintended for the manufacture of tyres, such as, for example,plasticizers or extending oils, whether the latter are aromatic ornonaromatic in nature, pigments, protection agents, such as antiozonewaxes, chemical antiozonants, antioxidants, antifatigue agents,reinforcing resins, methylene acceptors (for example, phenolic novolakresin) or methylene donors (for example HMT or H3M), as described, forexample, in Application WO 02/10269 (or US2003/0212185).

The examples which follow illustrate the invention without, however,limiting it.

In the examples, the rubber compositions are characterized after curingas indicated below.

Hysteretic Losses:

-   -   These losses, denoted P60°, are measured at 60° C. in        percentage. The strain for the losses measured is 35%.

Tensile Strengths and Elongations at Break:

-   -   The tensile strengths (in MPa) and elongations at break (in %)        are measured at 60° C.

Dynamic Properties:

-   -   The dynamic properties G* and tan(δ)_(max) are measured on a        viscosity analyzer (Metravib VA4000) according to Standard ASTM        D 5992-96. The response of a sample of vulcanized composition        (cylindrical test specimen with a thickness of 2 mm and with a        cross section of 78.5 mm²), subjected to a simple alternating        sinusoidal shear stress, at a frequency of 10 Hz, under the        standard temperature conditions according to Standard ASTM D        1349-99, is recorded. A peak-to-peak strain amplitude sweep is        carried out from 0.1% to 50% (outward cycle) and then from 50%        to 1% (return cycle). The results made use of are the complex        dynamic shear modulus (G*) and the loss factor, tan(δ). The        maximum value of tan(δ) observed (tan(δ)_(max)) is shown for the        return cycle.

In the examples which follow, the rubber compositions were prepared asfollows.

The compositions are manufactured in appropriate mixers using twosuccessive preparation phases well known to a person skilled in the art:a first phase of thermomechanical working or kneading (sometimesdescribed as “nonproductive” phase) at high temperature, up to a maximumtemperature (denoted T_(max)) of between 110° C. and 190° C., preferablybetween 130° C. and 180° C., followed by a second phase of mechanicalworking (sometimes described as “productive” phase) at a lowertemperature, typically of less than 110° C., for example between 60° C.and 100° C., finishing phase during which the crosslinking orvulcanization system is incorporated; such phases have been described,for example, in the abovementioned Applications EP-A-0 501 227, EP-A-0735 088, EP-A-0 810 258, WO 00/05300 or WO 00/05301.

In the process in accordance with the invention, the first(nonproductive) phase is carried out in two thermomechanical stages.During the first stage, all the necessary base constituents, theoptional additional covering agents or processing aids and various otheradditives, with the exception of the vulcanization system, areintroduced into an appropriate mixer, such as a normal internal mixer.This first stage is carried out at a temperature of between 110° C. and190° C. and preferably between 130° C. and 180° C. The second stage ofthermomechanical working is carried out in this internal mixer, afterintermediate cooling (cooling temperature preferably of less than 100°C.), in order to introduce the polysaccharide powders and also with theaim of subjecting the compositions to an additional heat treatment andin particular in order to further improve the dispersion, in theelastomeric matrix, of the reinforcing filler. The total duration of thekneading, in this nonproductive phase, is preferably between 2 and 10minutes at a temperature of less than or equal to 130° C., in order toavoid any damage to the polymer.

After cooling the mixture thus obtained, the vulcanization system isthen incorporated at low temperature, generally in an external mixer,such as an open mill; the combined mixture is then mixed (productivephase) for a few minutes, for example between 5 and 15 minutes.

The final composition thus obtained is subsequently calendered, forexample in the form of a sheet or of a plaque, in particular forlaboratory characterization, or else extruded, for example to form arubber profiled element used for the manufacture of semifinishedproducts, such as treads.

The vulcanization (or curing) is carried out in a known way at atemperature generally of between 130° C. and 200° C., under pressure,for a sufficient time which can vary, for example, between 5 and 90minutes, depending in particular on the curing temperature, thevulcanization system adopted, the kinetics of vulcanization of thecomposition under consideration or the size of the tyre.

Example 1

The aim of this example is to show the superiority of a tread inaccordance with the invention in comparison with a control tread in astarting situation on ice of a tyre, with a temperature of between −5°C. and 0° C.

The control tread is produced from a control composition A1 and thetread in accordance with the invention is produced from a composition A2in accordance with the invention, that is to say comprising a xanthangum powder in proportions and for a particle size corresponding to theinvention.

The compositions A1 and A2 have one and the same base formulation A.This base formulation A is as follows:

NR (1) 80 BR (2) 20 Carbon black N234 53 Paraffin 1 Antioxidant (3) 3Stearic acid 2 ZnO 3 Sulphur 1.1 Accelerator (4) 1.1 (1) NR: naturalrubber (peptized) (2) BR: polybutadiene with 4.3% of 1,2-; 2.7% oftrans-1,4-; 93% of cis-1,4-(Tg = −106° C.) (3)N-(1,3-dimethylbutyl)-N-phenyl-para-phenylenediamine (6-PPD) (4)N-cyclohexyl-2-benzothiazolesulphenamide (CBS)

The specific characteristics of the compositions A1 and A2 are given inthe following Table 1.

TABLE 1 Composition No. A1 A2 Xanthan gum (5) — 20 (5) Xanthan gum:Rhodopol 23, Rhodia (particle size: 106 μm)

Preparation of the Compositions

The composition A1 is manufactured with introduction of all of theconstituents onto an internal mixer, with the exception of thevulcanization agents (sulphur and accelerator), which are introducedonto an external mixer at low temperature (the constituent rollers ofthe mixer being at approximately 30° C.).

The composition A2 is manufactured in accordance with the process of theinvention, with introduction, during the first stage of thenonproductive phase, of the constituents onto an internal mixer, withthe exception of the xanthan gum, the said xanthan gum being introducedduring the second stage onto the internal mixer with a temperature notexceeding 130° C. in order not to damage the polysaccharide powder. Thevulcanization system is subsequently introduced, during the second“productive” phase, onto an external mixer.

The treads corresponding to the compositions A1 and A2 are subsequentlymanufactured in accordance with the process described above in thedescription.

Tests

The tests were carried out on a tyre with dimensions 11R22.5 XDW “ICEGRIP” fitted to a 7.5*22.5 wheel of a “Scania SV81” lorry for thestarting tests and of an “SV79 TDH” lorry for the braking tests.

The measurements were carried out in the following way:

-   -   for the starting: the distance traveled during the first two        seconds following starting is measured, starting being carried        out with a remotely set constant engine speed and the sudden        application of an engine torque and maintenance of this torque.

The measurements are carried out for the unladen vehicle and for theloaded vehicle, that is to say with a nominal load.

-   -   for the braking: stopping distance of the lorry from a        stabilized speed of 30 km/h with ABS braking.

The results obtained are given in the following Table 2.

TABLE 2 Tyre with tread having composition No. A1 A2 Starting, ice,between −5° C. and 0° C., unladen 100 108 Starting, ice, between −5° C.and 0° C., loaded 100 105 Starting, snow, unladen 100 120

It is clearly apparent that the tyre corresponding to the treadcomposition A2 in accordance with the invention makes it possible tosignificantly improve the grip of the tyre on frozen ground.

Example 2

The object of this example is to compare the stiffness properties,tensile strength properties, elongation at break properties, and thelike, of several rubber compositions in accordance with the inventionwith those of a tread composition in accordance with the abovementionedstate of the art, that is to say comprising a water-soluble powder, suchas carboxymethylcellulose or polyvinyl alcohol, PVA.

The compositions were produced according to the procedure described inExample 1 for A2, A2 being identical to the composition of the same namein Example 1.

A1 the compositions denoted A2, E(PVA) and F(Carbo) have the baseformulation A specified in Example 1; they differ from one another intheir content of xanthan gum or water-soluble powder as follows:

-   -   A2 comprises 20 pce of xanthan gum, Rhodopol 23, Rhodia        (particle size: 106 μm)    -   E(PVA): 20 pce of PVA sold under the name Mowiol 4-88 by        Aldrich, PVA 86.7-88.7 mol % hydrolysed, degree of        polymerization 630, viscosity at 4% in H₂O at 20° C. 3-5 mPa·s        (particle size: 500 μm)    -   F(Carbo): 20 pce of carboxymethylcellulose, sodium salt,        originating from Fluka (No. 21903), characterized by a viscosity        in solution of 700-1500 mPa·s (at 1% in H₂O, at 25° C.) and by a        degree of substitution of 0.7-0.85, (particle size: 89 μm).

Table 3 gives the properties measured after curing at 150° C. for 25minutes.

TABLE 3 Composition No. A2 E (PVA) F (Carbo) Elongation at break (%) 488339.6 404.4 Tensile strength (MPa) 16 9.9 13.9 G* 1.77 1.96 1.92Tan(δ)_(max) 0.195 0.214 0.212

It is found that the compositions A2 in accordance with the inventionexhibit a much better compromise in mechanical reinforcement without amajor damaging affect on the hysteresis of the material than themixtures comprising polyvinyl alcohol, E(PVA), orcarboxymethylcellulose, F(Carbo).

This example thus allows us to illustrate the improvement in rubberproperties of the compositions in accordance with the invention incomparison with the compositions described in the prior art andconsequently the improvement in the wear resistance of the compositionsin accordance with the invention in comparison with these compositionsof the prior art. In point of fact, it is precisely this decline in thewear resistance for the compositions of the prior art which, as wasstated at the beginning of the description, rendered them unusableindustrially.

Example 3

The aim, identically to Example 1, of this example is to compare thedifferent rubber properties of a control diene tread composition notincluding polysaccharide or water-soluble powder with tread compositionsin accordance with the invention, that is to say comprising a xanthangum powder. The two compositions with the base formulation B exhibit ablend of natural rubber and of synthetic rubber different from that ofExample 1.

This base formulation B is as follows:

NR (1) 40 BR (2) 60 Silica (3) 80 Carbon black N234 5 Aromatic oil 65Antiozone wax 1.5 Coupling agent (4) 5 Antioxidant (5) 2 Stearic acid 1ZnO 1.5 DPG (6) 1.5 Sulphur 2 Accelerator (7) 1.5 (1) NR: natural rubber(peptized) (2) BR: polybutadiene with 4.3% of 1,2-; 2.7% of trans-1,4-;93% of cis-1,4-(Tg = −106° C.) (3) Silica “Zeosil 1165Mp” from Rhodia,type “HDS” (4) Coupling agent: TESPT (“Si69” from Degussa) (5)Antioxidant: N-(1,3-dimethylbutyl)-N-phenyl-para-phenylenediamine(6-PPD) (6) DPG: diphenylguanidine (Perkacit DPG from Flexys) (7)Accelerator: N-cyclohexyl-2-benzothiazolesulphenamide (CBS)

The specific characteristics of the compositions B1 and B2 are given inthe following Table 4:

TABLE 4 Composition No. B1 B2 Xanthan gum (8) — 20 (8) Xanthan gum:Rhodopol 23, Rhodia

The first kneading phase of the process (“nonproductive” phase) iscarried out in a mixing vessel. The main ingredients (elastomer, silica,oil and chemical additives) are introduced therein.

The vulcanization system is introduced in a finishing phase of theprocess (“productive” phase) carried out on an open mill.

Table 5 gives the properties measured after curing at 150° C. for 40minutes.

TABLE 5 Composition No. B1 B2 Elongation at break (%) 700 600 Tensilestrength (MPa) 17 12 P60° 21 19

It is found that the composition B2 in accordance with the invention,comprising a xanthan gum, has rubber properties in terms of elongationat break and of tensile strength which have deteriorated in comparisonwith the control composition; however, this deterioration is not of aprohibitory nature for a person skilled in the art. Furthermore, areduction in the hysteric losses is very surprisingly observed for B2 incomparison with the control composition B1, which signifies animprovement in the rolling resistance in a tyre tread application.

Thus, on the one hand, the foreseeable deterioration in the rubberproperties in terms of elongation at break and tensile strength, due tothe introduction of xanthan gum into the composition in accordance withthe invention, remains acceptable to a person skilled in the art and, onthe other hand, he observes an improvement with regard to the hysteresisproperties.

Example 4

The aim of this example is to demonstrate the superiority of a tread inaccordance with the invention in comparison with a control tread in astarting situation on ice of a tyre with a temperature of between −5° C.and 0° C. for treads manufactured in accordance with the processdescribed in detail in the description with the control rubbercomposition B1 and the rubber composition B2 in accordance with theinvention, which are identical to those of the preceding Example 3.

These compositions B1 and B2 were used for the manufacture of tyretreads with dimensions of 205/55R16 of the “Michelin X Ice” brand fittedto a “Toyota Pronar” vehicle having an antilock braking system (ABS) anda traction control system (TCS).

Running tests were carried out on ice, during which different types oficy tracks and temperature conditions were tested.

Braking Tests:

The results obtained after longitudinal braking (distance traveled inorder to decelerate from 20 to 5 km/h) are indexed with respect to thebraking distance of the control tyre. The braking tests are carried outon a sheltered ice track.

The braking performance index of the “test” tyres is obtained by thefollowing formula:

PIBraking=(Braking distance of the control/Braking distance of the testtyre)*100.

Starting Test:

The results obtained after the starting test (active traction controlsystem) correspond to the time necessary in order to accelerate from 5to 20 km/h at full engine speed under control of the TCE system.

The ice acceleration tests are carried out on a covered track.

The performance index for the starting of the “test” tyres is obtainedby the following formula:

PIStarting=(Starting time of the control/Acceleration time of the testtyre)*100.

The results obtained are given in the following Table 6:

TABLE 6 Tyre with tread having composition No. B1 B2 Braking, ice (−3°C.) 100 111 Braking, ice (−8° C.) 100 104 Starting, ice (−3° C.) 100 103Starting, ice (−8° C.) 100 105

It is clearly apparent that the tyre corresponding to the treadcomposition B2 in accordance with the invention makes it possible tosignificantly improve the grip of the tyre on frozen ground.

1. Tyre tread having a composition based on at least one dieneelastomer, one reinforcing filler and one vulcanization system, whereinthe composition comprises a xanthan gum powder in a proportion of 10 to40 parts by weight per hundred parts of elastomer, pce.
 2. Treadaccording to claim 1, in which the proportion of xanthan gum powderincluded in the composition is from 15 to 30 pce.
 3. Tread according toclaim 1, in which the size of the constituent particles of the xanthangum powder is between 10 and 500 μm.
 4. Tread according to claim 1, inwhich the size of the constituent particles of the xanthan gum powder isbetween 50 and 300 μm.
 5. Tread according to claim 1, in which thexanthan gum powder is used as a blend with one or more otherpolysaccharide powders.
 6. Tread according to claim 5, in which thexanthan gum powder is used as a blend with a guar gum powder.
 7. Treadaccording to claim 1, in which the diene elastomer is predominantly, formore than 50 pce, an isoprene elastomer.
 8. Tread according to claim 1,in which the reinforcing filler predominantly comprises carbon black. 9.Tread according to claim 1, in which the reinforcing fillerpredominantly comprises silica.
 10. Tread according to claim 1, in whichthe reinforcing filler comprises a blend of carbon black and silica. 11.Process for obtaining a rubber composition for a tread based on at leastone diene elastomer, one reinforcing filler and one vulcanization systemwhich comprises a first phase of thermomechanical kneading of theconstituents of the composition, with the exception of the vulcanizationsystem, wherein the composition comprises a xanthan gum powder in aproportion of 10 to 40 pce and in that the xanthan gum powder isincorporated during the first kneading phase.
 12. Process according toclaim 11, in which the proportion of xanthan gum incorporated is between15 and 30 pce.
 13. Process according to claim 11, in which the firstphase of the process is carried out in two stages: a first stage ofkneading the combined constituents of the composition, with theexception of the xanthan gum powder, and a second stage in which thexanthan gum powder is incorporated.
 14. Process according to claim 13,in which the first stage of the first kneading phase is carried out at atemperature of between 110° C. and 190° C.
 15. Process according toclaim 13, in which the first stage of the first kneading phase iscarried out at a temperature of between 130° C. and 180° C.
 16. Processaccording to claim 13, in which the second stage of the first kneadingphase is carried out at a temperature of less than or equal to 130° C.,after cooling on conclusion of the first stage.
 17. Process according toclaim 16, in which the cooling between the first and the second stagesof the first kneading phase is carried out to a temperature of less thanor equal to 100° C.
 18. Process according to claim 11, in which thesecond phase of the process is carried out at a temperature of between60° C. and 100° C.
 19. Tyre comprising a tread according to claim
 1. 20.Winter tyre comprising a tread according to claim
 1. 21. Tyre intendedto equip heavy-duty vehicles comprising a tread according to claim 1.22. Tyre intended to equip passenger vehicles comprising a treadaccording to claim 1.